1. Field of the Invention
The present invention relates to a pneumatic tire, improved in the durability and the lightening of weight.
2. Description of the Prior Art
The applicant of this invention has proposed a tire improved in the bead durability and in lightening of weight in Japanese Patent Application Laid open No.10-193924 (U.S. Pat. No. 2,837,840). FIG. 5 is a schematic cross sectional view of a tire for heavy load in the prior art, showing the outer contour from a side wall portion (a) to a bead portion (b). The contour of the conventional tire in this region had been formed as an arc (c), connecting a tire cross section maximum width point p1 and a flange leaving point p2, as shown by a dotted line in the figure. On the other hand, the tire proposed in the Japanese Patent Application comprises a recessing portion (d) recessing inward than the norm arc (c) with respect to the tire axis direction, the recessing portion is disposed adjacent to the flange leaving point p2.
In the tire proposed by us, the volume of the gum is reduced by making a recessing portion (d) so that the deformation can take place in a large volume of the tire. Thus, the shear strain can be dispersed in a large volume of the tire, and the heat generation due to the deformation is made small. As a synergy of these effects, the bead durability is remarkably improved.
The tires proposed by us as well as a conventional tire have a contour of an arc (c) at the outer end of the carcass folded portion (e). As a result, there can be a case that the thickness (t) of the gum is too thin, because of the uncontrollable change of the side wall gum thickness due to the tolerance in the fabrication process or the tolerance of the height of the carcass folded portion (e). When the gum thickness (t) is too thin, a so-called gum destruction can occur due to the shear stress at the outer end of the carcass folded portion (e), which will be caused by the deformation of the side wall (a) due to the repeated inflation and the rolling of tire under a load, namely, such a structure of tire can lead to a serious tire damage, which could not be mended.
Therefore, a minimum necessary thickness (t) of the gum must be secured to avoid such a tire destruction. And it was forced to use a side wall gum material, which is thick as a whole. However, when such a thick gum material is used in a tire proposed by us in the former Japanese Patent application, the aforementioned advantages of such tire, i.e., the lightening of the bead portion and the improvement of the durability, can not be attained.
By the way, when the thickness (t) of the gum material is thickened locally, the carcass body will be pressed inwardly by the gum and extend along a path deviated inwardly from the optimum path line of the carcass. In such a case, the bead durability will be remarkably deteriorated, contrary to the intention.
An object of the present invention is to propose a pneumatic tire, the minimum necessary thickness (t) of the gum of which can be secured, restricting the increase of the gum volume to be small, so that the advantages of the lightening of weight and the improving of durability can be more effectively attained.
According to one aspect of the present invention, a pneumatic tire comprises a carcass having a carcass body portion extending from a tread portion to a bead core through a side wall portion, and a folded portion folded at the bead core, which is continuously formed with the carcass body portion, wherein:
in a standard state that the tire is installed on a normal rim and the inner pressure of the tire is the normal, the contour of the tire outer surface in the meridian cross section between a tire cross section maximum width point P1, which is the most outer point of the tire surface in the tire axis direction, and a flange separating point P3 to leave from a flange of the rim, is defined as follows:
the contour includes a substantially triangular formed projecting portion projecting outside in the direction of the tire axis direction with respect to a norm arc, which is an arc having its center on a line passing the tire cross section width maximum point P1, the norm arc passes through the point P1 and tangentially contacts with the flange, and a recessing portion recessing in the tire axis direction toward inside with respect to the norm arc, the recessing portion extends toward the inner side in the radial direction continuously with the projecting portion;
the height (h1) of a maximum projecting point with respect to a bead base line is 0.85xcx9c1.15 times of the height (h0) of the outer end of the folded portion, where the maximum projecting point is a point on the projecting portion and is separating at most from the norm arc in the tire axis direction,;
the height (h2) of the maximum recessing point with respect to a bead base line is smaller than the height (h1), and is 0.30xcx9c0.90 times of the height (h0), where the maximum recessing point is a point on the recessing portion and is separating at most from the norm arc in the tire direction; and
the maximum boss distance (q1) from the norm arc to the maximum projecting point and the maximum recess distance (q2) from the norm arc to the maximum recessing point are respectively 0.001xcx9c0.040 times of the tire cross section width (W).
Wherein, said normal rim is the xe2x80x9cstandard rimxe2x80x9d specified in JATMA, the xe2x80x9cMeasuring Rimxe2x80x9d in ETRTO, the xe2x80x9cDesign Rimxe2x80x9d in TandRA or the like., and said normal inner pressure is the xe2x80x9cmaximum air pressurexe2x80x9d in JATMA, the xe2x80x9cInflation Pressurexe2x80x9d in ETRTO, the maximum pressure given in the xe2x80x9cTire Load Limits at Various Cold Inflation Pressuresxe2x80x9d table in TandRA or the like.